Spray pistols and nozzles for spraying different kinds of coating materials are well known. The most common sprayed coating material is paint, but even other materials such as vinyls are used.
When spraying paint and other coating materials it is of great importance that the work piece to be sprayed receives as even a coating as possible. This is for example the case when spray-painting car chassis, whereby spray-painting is carried out automatically or semiautomatically along an assembly line and is often performed by a robotic system. Since spray systems for different types of coating materials are principally the same, for the sake of simplicity, and without lessening its generality, the description below will refer to the spraying of paint.
Known paint spraying systems normally include a device for supplying paint which includes some form of pump, compressor, or pressure vessel for feeding paint under pressure; a paint pressure regulator or other actuator; and a spray nozzle, which is connected to the pressure regulator by means of a hose. Several factors contribute to the difficulties these known systems suffer from when it comes to ensuring that the paint coating of the work piece will be even.
An important factor which makes regulation of the flow of paint through the paint nozzle more difficult is the viscosity of the paint. Even in the presence of a constant supply pressure, a change in the viscosity leads to a change in the flow: The flow decreases when the viscosity increases, and increases when the viscosity decreases. Changes in viscosity may arise in different ways, e.g., as a result of the aging of the paint, the mechanical working of the paint and solvents in a circulation system, or the deliberate addition of solvents. The viscosity also decreases with increasing temperature and increases with decreasing temperature.
Another obvious factor which may influence the paint flow through the nozzle is a deliberate or inadvertant change in the supply pressure. Of course, no pump or compressor can achieve completely constant supply pressure, but even such an ideal pump would not be sufficient to maintain constant pressure to the other system components since elastic hoses are as a rule used for transporting the paint. The elasticity of the hoses causes the hoses to act as accumulators, with drops and build-ups of pressure when the pistol is activated and shut off.
The most obvious factor is that the spray system often needs to be able to change the paint flow in order to maintain evenness without unnecessary paint losses. An example of this is when the spray system must alternate between spraying a wide portion and a narrow portion of the work piece. If the flow of paint through the nozzle were to remain constant, the layer of paint on the narrow portion would be thicker if only the width of the paint jet were changed without changing the flow. If the width is not changed, the thickness might be correct on the narrow portion, but paint would be wasted and adjacent portions might be sprayed inadvertently. If the time between spray-painting the wide and the narrow portions must be short, the further problem arises that the time for readjustment of the paint spray system must also be very short, since any delay leads to unevenness.
Different types of controllable paint pressure regulators are known, but the most common type comprises a housing which is divided into three chambers. Paint under pressure is fed into an intake chamber. A central chamber has an outlet which leads to a spray nozzle via a hose. Paint entering the intake chamber can pass through a closable opening in a dividing wall between the intake chamber and the outlet chamber. The third chamber consists of a compressed-air chamber which is separated from the outlet chamber by means of an elastic membrane. A pin or cone is securely joined with the membrane, extends through the outlet chamber, and partially plugs the closable opening. The pin usually has a conical end portion, which lies in the closable opening. By changing the air pressure in the compressed-air chamber, which determines the deflection of the membrane, the position of the end portion of the pin in the closable opening is changed and leads in turn to a change in the size of the area of the closable opening through which paint can flow.
Two main methods are used to generate a control signal to the paint pressure regulator. According to the one method, one feeds back a signal corresponding to the rate of paint flow. This is a natural solution considering the fact that it is the flow which is to be regulated. The flow is most often measured between the feed pump and the paint pressure regulator. Because of the accumulating effect of the hoses it would be preferable to measure the flow at the paint nozzle, but flow meters are generally too unwieldy for this. An additional disadvantage of feeding back the flow is that paint can spray from the nozzle even when the flow at the point of measurement is zero. This is because of the elasticity of the membrane in the paint pressure regulator, and because of the accumulating effect of an elastic hose; an overpressure may be present in the feed hose to the nozzle even when it is completely closed off, and if the regulation system does not compensate for this overpressure, the paint will be sprayed very unevenly immediately after the nozzle is once again opened.
According to the other known method one feeds back a signal corresponding to the paint pressure somewhere in the system. An advantage of this method is that pressure transducers are generally faster and less complicated that flow transducers; slow measurement causes slow regulation, which is very disadvantageous, particularly in paint spray systems which must rapidly change the rate of flow. The advantages of pressure measurement were mentioned above. Pressure measurement is, however, not wholly satisfactory. First, the relationship between pressure and flow varies as a function of the viscosity of the paint. Second, because of unavoidable inertial sluggishness in the supply device, and also because of viscous forces, a certain overpressure is required before any flow can arise at all.
The principle object of almost all control systems for paint spraying is to maintain the true rate of paint flow through the nozzle--the true flow--equal to a predetermined set-point value. U.S Pat. Nos. 4,019,653; 4,324,366; 4,487,367; 4,614,300; and the German Patents DE 34 23 094 A1; and DE 28 19 302 B2 disclose control systems for spraying paint In these systems either the flow or the pressure is fed back, but not both of these values, and none of them includes any device which compensates for changes in the viscosity of the paint.
U.S. Pat. No. 4,562,088, however, discloses a control system which measures not only pressure but also flow, and which takes viscosity into account. In this system the pressure is, however, measured at a point before the paint pressure regulator and only for the purpose of calculating the relationship between the temperature and the flow of the paint, and several temperature transducers are arranged in order to determine the necessary control parameters. Futhermore, the system relies on a method and a device in which paint must be drawn from the main flow and heated. Although the system represents in principle an improvement over the above-mentioned patents, it still cannot compensate for changes in supply pressure and other temperature-independent factors. Considering that it also requires a parallel paint conduit, in which choke regions, temperature transducers, and a heater are required, and also requires a microprocessor with its necessary peripheral equipment for calculating the exponential function which is used, this represents a very complicated system for ascertaining viscous effects.